1. Field of the Invention
This invention relates to sails and methods for their fabrication and is more particularly concerned with improvements in reinforced sails and methods for their preparation.
2. Description of the Prior Art
Significant advances have been made in recent years in the construction of lightweight sails having high stretch resistance and strength and particularly adapted for use in racing competitions. Sailcloth representative of the present state of the art is designed to have extremely high stretch resistance and strength in one principal direction and yet possess sufficient stretch resistance and strength in other directions to permit handling of loads imposed on the sail by stresses in directions other than the aforesaid principal direction. The sailcloth of this type is assembled into a sail by uniting a series of panels of the cloth in such a way that the principal direction or axis of the cloth which possesses the high stretch resistance and strength is oriented in the direction in which the major stresses will be placed on the sail when in use.
Sailcloths having the above characteristics are prepared in general by bonding together in the form of a laminate, a stretch resistant film of synthetic polymer such as the polyester (polyethylene terephthalate) film which is available under the trademark Mylar from DuPont Company, and a substrate which takes the form of a substantially orthogonal combination of warp and fill yarns in a scrim taffeta or warp knit. One of the courses of yarns (warp or fill) in this substrate is intended to bear the major load which will be exerted on the finished laminate and therefore consists of a high stretch resistant and high tensile modulus fiber such as a high tensile modulus polyester or polyamide. A particularly desirable fiber is that fabricated from the aromatic polyamide (also known as an aramid) available from the DuPont Company under the trademark Kevlar. The other course of yarn serves to hold the fabric together (except in the case of warp-knits where the knit yarn performs this function) as well as providing tear strength, burst strength and seamability (i.e. the ability of the resulting cloth to be joined to abutting strips by means of stitched and/or adhesively bonded seams).
The layers of material in the above laminates are bonded to each other throughout their entire interfaces by means of synthetic resin adhesives such as polyurethanes, cross-linked polyester thermosets and the like.
A particular example of the above type of laminate is that described in U.S. Pat. No. 4,444,822 in which a film of stretch resistant synthetic resin film is bonded to an unwoven, warp-knit scrim. The latter comprises a system of spaced parallel strands, the strands in one layer crossing the strands in the other layer and the crossing strands in the two layers being tied together by means of a knit strand. The strands are fabricated from polymeric fibers such as polyester, polyamides and the like.
However, the use of such warp-knit scrims and other related warp-knit or woven fabrics gives rise to a number of problems. The over/under yarn pattern present in taffetas or scrims or the misalignment of the tie yarns in warp-knits can give rise to crimps in the load bearing yarns of the resulting laminates, thereby detracting from the desired stretch resistance and strength in the load-bearing axis of the laminate. Further, the warp-knits or woven fabrics produced from high stretch resistant fibers such as those from polyesters and polyamides, especially the preferred polyamides such as the aromatic polyamides of which that available under the trademark Kevlar is particularly preferred, are relatively expensive to produce and add significantly to the cost of the laminates into which they are incorporated.
In my earlier U.S. Pat. No. 4,679,519 issued July 14, 1987, there is described a flexible reinforced laminate which is free from the various drawbacks set forth above and has proved eminently satisfactory in the fabrication of many types of lightweight sails having high strength and stretch resistance in predetermined directions in the sail as well as imparting stretch resistance and tear strength in other directions in the sail.
Various other means of providing lightweight reinforced sails have been described recently. Illustratively, Conrad U.S. Pat. No. 4,593,639 describes a method of constructing a sail which comprises applying reinforcing members to the surface of an appropriate skin (constructed as a sail in conventional manner from a film of Mylar or like material). The reinforcing members, which can be threads of stretch resistant material such as Kevlar are aligned in the direction of the principal stresses which will be sustained by the sail when exposed to the force of the wind. Variations of this concept are described in related Conrad U.S. Pat. No. 4,624,205, which issued on a continuation-in-part of the application on which the '639 patent issued, and in related Conrad U.S. Pat. No. 4,702,190 which issued on a continuation-in-part of the application on which the '205 patent issued.
Conrad U.S. Pat. No. 4,708,080 shows a sail fabricated from a plurality of individual panels, each of which is fabricated from a laminate having at least two layers of material such as Mylar film between which are disposed non-woven, force-bearing threads of Kevlar and like stretch resistant polymeric material, which threads are aligned along the principal stress lines to which the particular panel will be subjected when installed in the sail and subjected to the force of the wind.
The present invention, which is an improvement of that of my '519 patent, is directed to a composite sail which has greatly enhanced stretch and tear resistance in the direction of the principal stress lines which may occur in a direction toward the foot of the sail, or in a direction along the foot of the sail, or otherwise. The composite sails of the present invention are of particular value for use in racing yachts and provide highly efficient, light weight sails having significantly extended useful life compared with sails hitherto available.